The present invention relates to a seatback frame for a vehicle seat and, more particularly, to a seatback frame for a vehicle seat equipped with a seat belt of the three-point type wherein a retractor for winding a seat belt is mounted at a lower portion of the seatback and allows a shoulder belt to be pulled out for restraining a seat occupant, a belt-through member is mounted at an upper distal end of the seatback for guiding the shoulder belt, fixture segments are mounted at both sides of a seat cushion for supporting both ends of a lap belt which restrains the waist of the seat occupant, and a distal end of the shoulder belt is supported with one of the fixture segments.
In general, seat belts in practical use are divided into two types, a first type in which a retractor is fixedly mounted to a pillar of a vehicle body and a second type in which the retractor is directly fixed to a seatback frame. The seat belt in normal use is constructed of a lap belt portion for restraining the waist of a seat occupant and a shoulder belt portion for restraining an area covering the shoulder and the waist of the seat occupant, which are arranged in a three-point support structure which includes lap anchor points located at both sides of a seat cushion and a shoulder anchor point located at an upper distal end of a seatback.
Since the vehicle seat, wherein the shoulder anchor point is located on the seatback frame, encounters a significantly large impact shock delivered from the shoulder belt and acting on the shoulder anchor point, it is required for the seatback frame to be reinforced. An example of such a seatback frame with a reinforced structure is exemplarily shown in FIG. 5. As shown in FIG. 5, the seatback frame 1 is comprised of a pipe frame 2 which is shaped to conform along an external shape of a seatback, and a tower frame 3 on which a belt-through member (not shown) for a shoulder anchor point is mounted. To provide a reinforced structure for the tower frame 3, particularly, a cross member 4 is obliquely placed so as to extend between an upper distal end of the tower frame 3 and a bracket 2a opposed thereto and welded to both of these components.